To click on links in the alternate screen buffer, it is necessary that
the `shift` button is held down, otherwise mouse events are captured by
the application. However this would also require that `Shift` is added
to the `mouse.url.modifiers` option. Thus it is not possible anymore to
click on URLs unless the shift button is always held down.
To resolve this issue, the matching of modifiers has been relaxed. If a
modifier is specified in the config, it is always required to be held
down. However if a modifier is held down which is not specified, it is
still accpeted. This one-way relaxed matching allows clicking on links
with or without shift held down without having to make use of the
`mouse.url.modifiers` setting at all.
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